This invention relates generally to terminator devices. More specifically, the present invention relates to a switchable terminator apparatus and a method for using the apparatus for disconnecting terminating resistors on a device using a differential transmission bus.
To transmit signals reliably on a differential transmission bus, the bus must be terminated with an array of resistors at both ends, and only at the ends. Many devices are manufactured with these terminating resistors located on the device. This presents a problem with the devices that are not at the ends of the bus, thus the terminating resistors must be removed from these devices.
For example, peripheral devices connected to a computer may include disk drives, compact disk drives and output printers. The interface between the computer and the peripheral device is generally characterized by a bi-directional cable carrying high speed logic signals. All signals are transmitted via dedicated differential lines with an appropriate termination impedance.
Specifically, one application where reliable data transfer becomes important is in the field of data transmission on a fully loaded SCSI (acronym for small computer system interface) bus line for communication between a plurality of data transceivers and a central processing unit (CPU) in a small computer. A SCSI system has a transmission line over which a plurality of units coupled to the transmission line may communicate. A terminator is coupled to the two physical ends of the transmission line.
It is necessary to provide a terminating impedance on each such line which matches the characteristic impedance of the connecting cable used, in order to avoid line reflections. This termination is generally provided by means of a 330 ohm resistor connected to a ground, a second 330 ohm resistor connected to a voltage source, and a 150 ohm resistor connected to both of the 330 ohm resistors.
Further, in a system where only one peripheral device is connected to the transmission line, that peripheral device must also have such a termination network in order to avoid line reflection problems. Where a multiplicity of peripheral devices are connected to the same transmission line, then only one of the peripheral devices need have a termination network. The effect of paralleling several termination networks would reduce the effective terminating impedance to an incorrect value and would place an excessive load on the logic drive element.
In most present disk drive designs, a termination having a socket connected to each line to be terminated is used. The common points where the 150 ohm resistor connects to both of the 330 ohm resistors can be plugged into the socket. Also, the socket provides that the end of one 330 ohm resistor is connected to ground and one end of the other 330 ohm resistor is connected to an appropriate power supply. To connect the termination network to a particular peripheral device, an appropriate resistor array is plugged into such socket. If no termination network is required, the resistor array is omitted. In practice, this is provided by using a resistor array and a matching socket.
However, many peripheral device designs (for example, in some disk drive designs), space or other considerations demand that access to the printed circuit board containing the termination package be restricted. The circuit board, for example, may be mounted with the components facing away from the outside toward some other part of the disk drive structure. In such circumstances, it is thus necessary to remove or to partially dismantle the disk drive in order to gain access to the termination network. Some disk drive designs have attempted to resolve this problem by utilizing an angled socket so that, using appropriate tools, such as needle nose pliers, a termination package can be removed or inserted from the exposed edge of the board.
Thus, the prior art teaches mechanical disconnections by physically disconnecting the cables from the devices.
Accordingly, there has arisen a need in the computer equipment industry for an improved terminator apparatus and method for disconnecting the terminating resistors on a device using a differential transmission bus.